Nicotine addiction is a significant medical and socioeconomic burden on society. Although numerous treatment strategies have been employed to encourage cessation of smoking, relapse continues to represent a major challenge to those efforts. Surprisingly, relatively little is known about gender-specific influences on nicotine use, relapse, and treatment responsiveness. Studies using animal models have provided valuable information about biological and environmental factors that contribute to nicotine addiction;however, these studies have primarily focused on male subjects. Accordingly, this proposal will focus on identifying sex- related and estrous cycle-dependent differences in nicotine self-administration, vulnerability to relapse, and sensitivity to pharmacotherapies that are clinically used as smoking cessation treatments. More specifically, established mouse models of operant self-administration will be employed to study the reinforcing effects of nicotine, and reinstatement procedures will be used to examine the ability of nicotine, conditioned cues, and stress to trigger reinstatement of nicotine-seeking behavior in male and female mice. Additionally, the effects of pharmacological drugs (buproprion and varenicline) on nicotine self- administration, as well as their ability to block or attenuate reinstatement of nicotine-seeking behavior will be evaluated. To examine estrous cycle-related differences in relapse vulnerability, reinstatement testing will be conducted in separate groups of females to coincide with the estrus and non-estrus phases of the cycle. Similarly, to examine estrous cycle-dependent differences in responsiveness to the pharmacotherapeutic agents, drug treatments will begin at different estrous cycle phases in females. Progesterone levels will be measured in females and correlated with behavioral measures of nicotine's reinforcing effects, as well as reinstatement of nicotine-seeking provoked by various trigger stimuli. As such, these studies not only address the overarching theme of the SCOR, but also fill a critical void in the literature regarding this important clinical issue. This project directly relates (by design) to a clinical SCOR project focused on investigating the impact of timing of pharmacological treatment within the menstrual cycle on relapse to smoking in women (Project #4). Results from this project will also interface with preclinical (Project #1) and clinical (Project #2) SCOR projects that address similar gender-related issues regarding cocaine dependence and relapse. Overall, the complementary nature of the preclinical and clinical research components of the MUSC SCOR Center will foster and facilitate translational efforts to guide the development of new and more gender-specific treatment strategies for nicotine (and other) drug addiction and relapse in women and men.